608 
38 C3 
>y 1 G-160 



ANGULAR-LEAFSPOT OF CUCUMBER: DISSEMINATION 
OVERWINTERING, AND CONTROL 



BY 



EUBANKS CARSNER 

____ BI1 



Reprinted from JOURNAL OF AGRICULTURAL RESEARCH 

Vol. XV, No. 3 : : : : Washington, D. C, October 21, 1918 




PUBLISHED BY AUTHORITY OF THE SECRETARY OF AGRICULTURE, WITH THE COOPERATION 
OF THE ASSOCIATION OF AMERICAN AGRICULTURAL COLLEGES AND EXPERIMENT STATIONS 



WASHINGTON : GOVERNMENT PRINTING OFFICE : 1918 



l 



ANGULAR-LEAFSPOT OF CUCUMBER: DISSEMINATION, 
OVERWINTERING, AND CONTROL 

COOPERATIVE INVESTIGATIONS BETWEEN THE UNIVERSITY OF WISCONSIN AND 
THE BUREAU OF PLANT INDUSTRY, UNITED STATES DEPARTMENT OF AGRI- 
CULTURE 

By Eubanks CarsnEr 1 

Formerly Heinz Fellow in Plant Pathology, University of Wisconsin, and Collaborator, 
Office of Cotton, Truck, and Forage Crop Disease Investigations, Bureau of Plant 
Industry, United States Department of Agriculture 

INTRODUCTION 

The bacterial nature and the symptoms of the angular-leafspot of 
cucumber (Cucumis sativus) have been clearly described by Smith and 
Bryan (15). 2 Prior to this paper the disease had been reported in this 
country by Burger (1-4) and from Europe by Traverso (16) and Poteb- 
nia (12). The two latter writers accepted Burger's statement that the 
organism which caused the spots on the leaves was also responsible for 
a serious rotting of the fruit. The inoculation studies made by Smith 
and Bryan (13) showed that the bacterium which caused the leaf -spotting 
was unable to produce a soft-rotting of the fruit. Extensive tests by 
the writer have confirmed their finding in this regard. 

The bacterial cause of the disease was determined independently by 
the writer in the summer of 191 5, as set forth in a prehminary note (5). 
The morphological and physiological studies which were subsequently 
made of a strain of the causal organism isolated from a Wisconsin speci- 
men gave results essentially in agreement with those reported by Smith 
and Bryan (15). The name given to the organism by these writers is 
"Bacterium lachrymans." According to Migula's system of classification 
it would be called "Pseudomonas lachrymans." 

The damage caused by the angular-leafspot can not be accurately 
estimated. It varies greatly with differing weather conditions, but enough 
weather favorable for the disease prevails each year to make the injury 
of considerable importance (Pi. 13, A). The writer's ideas as to the 
destructiveness are based mainly on his field experience in Wisconsin 
and adjoining States during three summers, together with more limited 
observations in Virginia and southern California. 

It is when the disease appears in a field early in the summer that the 
greatest damage results, as would naturally be expected. Young plants 

1 The writer wishes to express his appreciation to Dr. L. R. Jones, of the Wisconsin Experiment Sta- 
tion, for helpful interest and advice in the prosecution of the work and to thank Mr. W. W. Gilbert and 
Dr. M. W. Gardner, of the Bureau of Plant Industry, for helpful suggestions and cooperation. 

2 Reference is made by number (italic) to "Literature cited," p. 220. 



Journal of Agricultural Research, Vo1 - XV, No. 3 

Washington, D. C (201) ° ct - «• J 9 l8 

pw KeyNo.G-160 



202 Journal of Agricultural Research vol. xv, No. 3 

are often so severely attacked that stunting results (Pi. 13, B). A few 
scattered plants in a field or nearly all may be so affected, depending 
chiefly on the meteorological conditions. Some observations on the 
extent of the injury by angular-leaf spot in a representative locality 
may here be noted to give a more definite idea of the damage which 
it causes. At Ripon, Wis., in the summer of 1914, sixteen cucum- 
ber fields were under observation. The disease appeared in seven 
of these while the plants were in the seedling stage, and by the middle 
of the season, August 1 1 , it had resulted in the severe spotting of approxi- 
mately 25 per cent of the leaves. On August n the disease was also 
present in three of the nine other fields, but in these it had been intro- 
duced only a short time and had not yet become generally distributed. 
A survey of the same locality on August 15, 191 5, indicates how wide- 
spread the disease may become, especially when the fields are close 
together. On that date 28 out of 35 fields visited were found to be 
infested. A later visit revealed even further distribution. 

The losses in the regions where cucumbers are grown for pickling 
purposes result mainly from the decrease in yields due to the destruc- 
tion of leaf surface, but it seems quite probable that in other sections, 
as has been pointed out by Burger (r, 2) for Florida, where cucumbers 
are grown for "slicing" purposes, and so must be shipped to distant 
markets or kept in storage for considerable lengths of time, an addi- 
tional loss may come from the secondary soft-rotting of the fruit. Lim- 
ited observations by the writer indicate that in California the soft- 
rotting of the fruit as an indirect result of the angular-leafspot may 
cause some loss. The bacterium causing the leafspot does not directly 
cause the fruitrot, but through the wounds which it makes on the fruit 
softrot organisms are frequently able to gain entrance. 

The widespread distribution of angular-leafspot and its frequent 
occurrence give it a place among the major diseases of the cucumber. 
The aggregate loss which it entails probably exceeds that caused by some 
of the other diseases which are more destructive in limited areas. In 
America this disease has been reported from Florida by Burger (1-4), 
and from Connecticut, Indiana, Michigan, New York, and Wisconsin 
and the Canadian Provinces of Ontario and Quebec by Smith and Bryan 
(15). To this list of regions v/here it is known to occur may now be 
added California, Colorado, Illinois, Iowa, Minnesota, and Virginia. 
That it is probably widely distributed in Europe is indicated by the fact 
that Traverso (16) reported it from Italy, and Potebnia (12) recorded its 
occurrence in Russia. The wide distribution of the disease is a fact that 
should be expected in view of the evidence to be presented that the 
causal bacteria are seed-borne and in view of the general occurrence of 
the trouble in seed -growing localities. 



Oct. 2i, igis Angular-Leaf spot of Cucumber 203 

PRELIMINARY CONSIDERATIONS 

The chief purpose of this paper is to present evidence bearing on the 
phases of the problem which are of direct economic significance. Certain 
other parts of the work which has been done have yielded results worthy 
of record; and, since some of these results are pertinent to the questions 
of dissemination, overwintering, and control, they may appropriately be 
presented before passing to the consideration of the latter points. 

desiccation 

Many questions in regard to the dissemination and overwintering of 
the causal organism of angular-leafspot depend on its sensitiveness to 
desiccation. The organism has been shown by repeated tests to be 
relatively sensitive to drying on glass. With a 3-mm. platinum loop 
drops were transferred to carefully cleaned cover glasses from 36-hour 
cultures in beef bouillon and in cucumber-leaf decoction from the leaf 
exudate and from a suspension in distilled water of the organisms from 
freshly invaded tissue. None of these showed viable organisms after 
four days' drying at room temperature. Smith and Bryan {15, p. 470) 
found that the organisms from a young bouillon culture when dried on 
glass were viable after 2 1 days. The variance in these results may possi- 
bly be due to some slight difference in methods, which may have made a 
difference in the time for which. the bacteria were exposed to complete 
desiccation. 

Freshly invaded fruit and leaf tissue dried in diffuse light at room 
temperature showed viable organisms after 3 and 10 days, but none 
were alive after 32 days. 

Short periods of drying, four to five days, resulted in the death of all 
organisms on seed which had been disinfected with mercuric chlorid, 
washed thoroughly, and then wet with a young bouillon culture of 
Bacterium lachrymans. The fact, however, that the bacteria do survive 
long periods of desiccation on or in the seed is shown by the evidence to 
be presented under the discussion of overwintering. 

The results of one test on culture media are here pertinent because 
they show that there are conditions under which the organisms may 
withstand long periods of drying. On February 2, 191 6, six tubes of 
potato-dextrose agar, in each of which had been suspended approximately 
0.5 gm. of powdered calcium carbonate, were slanted and inoculated. 
The purpose was to see if the life of the cultures might be prolonged by 
neutralizing with the carbonate the acid resulting from the growth of 
the organisms. In a dextrose-containing medium the bacteria ordi- 
narily make a rapid growth for a short time and then all die, so that the 
tubes become sterile, usually within 10 days. The tubes in this test 
were set away at room conditions and, because of the low relative hu- 
midity of the laboratory air, rapidly dried out. Before they dried com- 



204 Journal of Agricultural Research voi.xv.N05 

pletely, which required nearly two months, an abundant growth of the 
organisms had been made. On November 8, 1916, the dry remains of 
the agar, carbonate, etc., from three of the tubes were transferred to 
tubes of bouillon. Growth occurred in all three tubes thus inoculated, 
and in each case the identity of the organism was established by inoculat- 
ing cucumber plants. Two of the remaining three tubes were similarly 
tested on February 9, 191 7. Growth resulted i/i each case, and inocula- 
tions again proved that the clouding of the bouillon was due to the 
angular-leafspot organism. Since all other evidence is opposed to the 
possibility of the formation of spores, the writer is inclined to explain 
the survival of some of the bacteria in these tubes by assuming that they 
were protected from complete desiccation. 

THERMAL RELATIONS 

The thermal death point of the angular-leafspot bacterium is be- 
tween 49 and 50 C. Tests were made in 10-cc. portions of beef bouillon 
in 'thin-walled test tubes at 46 , 48 , 49 , 50 , 52 , and 55 . Ten min- 
utes' exposure at 46 must have killed a large proportion of the organ- 
isms, because growth in tubes so exposed was much slower in appearing 
than in the unheated controls. In each test some but not all of the tubes 
exposed at 49 C. showed no growth. In none of the tests did growth 
occur in tubes exposed at 50 or temperatures above that point. 

An interesting contrast between the relation of temperature to 
angular-leafspot and its relation to the bacterial-wilt of cucumber was 
brought out at Madison, Wis., in 191 6. The maximum temperature as 
recorded by the United States Weather Bureau there averaged 36. 7 C. 
(98 F.) for the five days July 26 to 30, inclusive. The highest tempera- 
ture at the Weather Bureau Observatory was 38. 3 C. (101 F.), but in 
direct sunlight and near the ground undoubtedly the temperature was 
higher. This unusually hot weather did not appreciably check the devel- 
opment of angular-leafspot, which reached its maximum development 
within about 10 days thereafter, but it practically stamped out the bac- 
terial-wilt. Smith (14, p. 209) accoun 3 for the bacterial-wilt having 
been found only in cool climates on the basis of the low thermal death 
point, 43 C, of the causal organism. 

The relation of temperature to growth in artificial media has been 
found to agree with the report of Smith and Bryan (75, p. 470), and so 
need not be given in detail. 

The sensitiveness of the bacteria to freezing was tested by exposing 
them in different media in glass test tubes outside a north window during a 
period of low temperatures in the winter of 191 6-17. Dilute suspensions 
of the bacteria in distilled water, freshly-inoculated tubes of beef bouillon, 
beef bouillon with approximately 2 per cent of sodium chlorid, and 24- 
hour agar slope cultures were exposed. During the first 9 days of the 



Oct. 21, 1918 Angular -Lea) spot of Cucumber 205 

exposure the highest temperature was o° C. (32 F.), the lowest — 25. 5 C. 
(-14 F.), and the average daily mean was — i5.5°C. (4.1 F.). One 
tube of each medium was taken in after 24 hours and longer periods and 
thawed slowly in cold water. In the salt bouillon all the bacteria were 
dead after 24 hours. In bouillon without salt all were dead after 60 
hours. No test was made of the suspension in distilled water after the 
60-hour interval, but no colonies developed in plates poured from one 
of the other tubes melted after 4 days. On the agar some of the organ- 
isms were alive after 6 days, but after 17 days all were dead. The sensi- 
tiveness to freezing was undoubtedly increased by the sodium chlorid 
in the bouillon. 

Smith and Bryan (75, p. 471) reported freezing the organisms for 15 
minutes in bouillon by means of salt and pounded ice. That exposure 
resulted in the death of nine-tenths of the bacteria. 

SENSITIVENESS TO GERMICIDES 

Tests of the sensitiveness of the organisms to formaldehyde, copper 
sulphate, and mercuric chlorid were made. The dilutions of formalde- 
hyde were made up by volume from the 40 per cent formaldehyde solu- 
tion known commercially as formalin. The copper-sulphate and mer- 
curic-chlorid solutions were made up 1 to 1,000 by weight and the 
desired dilutions made from these. Exposures were made in all cases 
by transferring a 3-mm. loop of a young bouillon culture to 10-cc. portions 
of the dilutions in vials floated on a water bath at 25 ° C. Tubes of 
melted agar were inoculated in duplicate or triplicate by a 3-mm. loop 
transfer from each vial after an exposure of 10 minutes. 

The test with formaldehyde resulted in the death of all organisms ex- 
posed to a dilution of 1 to 10,000, of nearly all in the 1 to 100,000 dilution, 
and of apparently none in the 1 to 500,000 dilution. The tests with cop- 
per sulphate and mercuric chlorid were repeated twice. With the copper 
sulphate the results did not agree throughout, but in all cases all organ- 
isms were killed or prevented from developing by the 1 to 100,000 dilution. 
There were no colonies, or a strikingly smaller number than from the con- 
trols, in the plates poured from the 1 to 500,000 dilution. All organisms 
were killed by exposure to dilutions of mercuric chlorid of 1 to 1 ,000,000. 
The sensitiveness of the organism to copper sulphate was tested by 
Smith and Bryan (75, p. 474). Their results show a slightly less marked 
sensitiveness to this chemical than was found in the tests made by the 
writer. The temperature at which their exposures were made was not 
stated. 

PLANTS ATTACKED 

Little attention had been previously given to the question of the host 
range of the disease or to the question of variations in susceptibility or 
resistance to the disease in the case of the different types of cucumbers. 



206 



Journal of Agricultural Research 



Vol. XV. No. 3 



Because of the bearing which these questions might have on distribution* 
overwintering, and control, 12 horticultural varieties of cucumbers and a 
large number of other cucurbits were tested as to susceptibility to angular- 
leafspot. The varieties of field cucumbers and the other species and 
varieties of cucurbits which are listed in the following table were grown 
in a cucumber field thoroughly infested with angular-leaf spot, where they 
were under the most favorable conditions for infection. The varieties of 
forcing cucumbers were tested by inoculation in the greenhouse. 



Plants exposed to angular-leaf spot infection 



PLANTS ATTACKED 



hor- 



Cucumber (Cucumis sativus), 
ticultural varieties: 

Davis Perfect. 

Chicago Pickling. 

Boston Forcing. 

Early Russian. 

Giant Pera. 

Japanese Climbing. 

Heinz Muscatine. 

Lemon. 

Thorburn's Everbearing. 

Rollison's Telegraph. 

Vaughan's Prolific Forcing. 

White Spine Klondyke. 
West Indian gherkin (Cucumis an 

guria). 
Mandera gourd (Cucumis acutangulis) 
Hedgehog gourd (Cucumis dipsaceus). 
Calabash gourd (Lagenaria vulgaris). 
Bryanopsis laciniosa. 



PLANTS NOT ATTACKED 

i. Balsam-apple (Momordica balsamina). 

2. Balsam-pear (Momordica charantia). 

3. Squirting cucumber (Momordica ela- 
terium). 

4. Pomegranate melon (Cucumis melo var. 
dudaim) . 

5. Cucumis grossulariaeformis. 

6. Muskmelon (Cucumis melo), 11 va- 
rieties. ° 

7. Snake melon (Cucumis melo var. flex- 
uosus) . 

8. Wild cucumber (Echinocystis lobata). & 

9. Watermelon (Citrullus vulgaris), 2 
varieties. ° 

10. Citron (Citrullus vulgaris). 

11. Japanese crookneck squash (Cucur- 
bita moschata). 

1 2 . Hubbard squash (Cucurbita maxima) . a 

13. Turban squash (Cucurbita maxima). 

14. Summer squash (Cucurbita pepo var. 
condensa). 

15. Pumpkin (Cucurbita pepo). a 

16. Gourd (Cucurbita pepo var. ovifcra). 

17. Trichosanthes colubrina. 

In the case of the plants other than cucumber attacked the causal 
organism was isolated from diseased spots and identified by inoculating 
cucumbers. Stained sections from paraffin-embedded material showed 
bacteria within the tissue of the leaf spots in all cases except that of the 
West Indian gherkin, which was unquestionably subject to the disease. 



STOMATAL, MOVEMENT AND INFECTION 

The fact that leaf infection took place through the stomata was re- 
ported by Smith and Bryan (75, p. 469), but the^/ gave no discussion of 
the conditions necessary for infection. Practicafly all of the earlier in- 
oculations made by the writer were performed in the evening, after dark, 

a Greenhouse inoculations aiso gave negative results. 

6 The wild cucumber plants were not in the experimental plots-, but grew near by and were artificially 
inoculated. 



Oct. 21, 1918 



Angular-Leaf spot of Cucumber 



207 



because the sensitiveness of the organism to sunlight was known and 
because moisture, such as dew on the leaves, was thought to be the most 
important factor in infection. A few infections wer-e nearly always ob- 
tained in this way, but the number was consistently smaller than occurred 
on leaves naturally infected. A suggestion that the factors limiting the 
number of leaf infections were in some way involved with the time of 
inoculation was obtained when, from a series of inoculations made in the 
field at intervals of 2 and 4 hours during the day and night of a 24-hour 
period, more abundant infections resulted from the inoculations made 
during the day. Evidence that infection occurs more abundantly when 
inoculations are made during the day was confirmed by other tests. 

The idea that duruig the process of photosynthesis enough oxygen was 
given off through the stomata to exert a chemotactic action on the causal 
bacteria was first conceived as a possible explanation of the different 
results from night and day inoculations. The hypothesis was abandoned 
after experimental tests. Plants which were kept in darkness for 24 
hours before and after inoculation became infected to about the same 
extent as the controls. 

The idea that stomatal movement might be a factor was next hit 
upon. Pool and McKay (11) found that there was a relation between 
stomatal movement in sugar-beet leaves and infection by Cercospora 
beticola. This fact suggested that in the case of the disease under con- 
sideration a similar relation might hold true. To study the behavior of 
the stomata the method described by Lloyd (10) of direct visual observa- 
tion of the stomata in situ was utilized. It was found that the stomata 
on the lower surfaces of the leaves were generally open during the day 
and closed at night. The movement of the stomata on the upper sur- 
faces was not always the same as those on the undersides, but this fact 
is of no special significance here. It was then found by repeated tests 
that inoculations on the under surfaces made in the morning, when the 
stomata were observed to be open, gave much more abundant infec- 
tions than did similar inoculations made at night, when the stomata 
were seen to be closed (PI. 14). The following table gives a comparison 
of the number of infections from night and day inoculations. The two 
plants used were of the same age, each having seven leaves at time of 
inoculation. Leaf 7 is the youngest leaf of each. They were treated 
similarly except for time of inoculation. 



Time inoculated. 


Leaf .5. 


Leaf 4. 


Leaf 5. 


Leaf 6. 


Leaf 7. 


7.?oD. m 




16 


O 

43 


5 
49 


48 
97 


45 
5° 







On the plant inoculated in the evening the youngest leaves, No. 6 and 
7, showed many more infections than did the older leaves, and this has 
been repeatedly found in other inoculations. Why this difference in in- 



208 



Journal of Agricultural Research 



Vol. XV, No. 3 



fection of leaves of different ages occurs is a matter of conjecture, but it 
is thought to be associated with the fact that younger tissues are more 
susceptible. Probably the relatively small number of organisms which 
retain their motility are able, when the stomata open, to establish them- 
selves in the younger leaves, but are not able to gain a foothold in the 
older tissues. 

The closure of the stomata may mechanically exclude the bacteria or 
may interfere with stimuli which attract them into the interior of the 
leaves. No attempt has been made to determine this point, but the first 
theory seems to the writer the more plausible. 

FRUIT INFECTION 

Fruit infection occurs naturally without wounds. Stomatal infection 
(fig. i) has been demonstrated in fruit artificially infected without 

wounding. Burger's (4) de- 
scription of the effect on the 
fruit is accurate in part, but 
the softrot which he empha- 
sizes results from organisms 
other than the species causing 
the small, circular, localized 
spots on the fruit, characteristic 
of angular-leafspot infections. 
The circular spots are at first 
water-soaked in appearance. 
Later their centers become 
whitened, owing to a cracking 

Fig. 1. — Cross section of epidermal portion of cucumber , . . . » ,« ,• 

fruit fixed eight days after inoculation with Bacterium and °- r y in g out - °* trie tlSSUeS 

lachrymans, showing presence of bacteria in stoma and (PI. 1 6, B). In fruit, as Well 

tissues below. 1 <• , • , , i_ 

as leaf tissue, the bacteria 
have been seen only in the intercellular spaces. 

DISSEMINATION 

The means by which the disease is spread have been given a good deal 
of attention because of the possible bearing which these might have on 
remedial measures. Some of the observations and experiments may throw 
light on other and similar bacterial diseases. 




BY RAIN AND WIND 

That the important relation of rainy weather to the progress of angular- 
leafspot, a factor previously observed, was principally in the dissem- 
ination of the causal organisms was made clear in the summer of 191 6. 
Healthy potted plants which had been placed outside of the greenhouse 
and at a distance of 4 feet from infected plants became diseased after a 



Oct 21, 1918 



Angular-Leaf spot of Cucumber 



209 



rainy period. Experimentally infection was secured by placing recently 
infected leaves on the ground beneath healthy plants on a day when 
there were frequent showers. In the fields at Madison newly infected 
spots appeared in abundance within five or six days after heavy rains, 
especially the rains of July 19 and August 3-5. Rain must fall at rela- 
tively frequent intervals to be effective in spreading the disease. Pro- 
longed rainless periods check the development of the disease to a great 
degree, especially if accompanied by high temperatures (PI. 15, C). 

The importance of rain in the development of the disease was clearly 
shown at Ripon, Wis., in 191 4. Owing to favorable rainy weather early 
in the season angular-leafspot spread throughout certain fields. Two 
of the infected fields at Ripon and one in a neighboring locality, which 
were visited on August 11 and 12, presented a striking appearance. 

The vines were so grown together as to nearly cover the ground, but 
the centers of the rows were clearly marked by the old, angular-spotted 
leaves in contrast with the healthy green of the later growth which had 
developed after the last heavy rain. 

Further evidence regarding the importance of rain in relation to the 
development of the disease was furnished by a comparison of conditions 
at Madison and Ripon, Wis., in 191 6. The striking difference in the 
amount and distribution of rainfall for the two places during the month 
of July can be seen in Table I. 

Table I. — Dates and amounts of rainfall at Madison and Ripon, Wis., in July, igi6 



Day of month. 


Precipitation (inches). 


Madison. 


Ripon. 


I 


0. OO 

•33 
.90 

1. 21 

.19 

•°3 
. OO 


O. 13 


12 


16 




19 




20 




22 




26 


. 02 




Total 


2.66 


26 







The time of planting and the earlier weather conditions were similar, 
and so it is highly probable that the disease appeared in both localities at 
about the same time, noted first at Madison on July 3. At the end of 
the month the disease was widespread and affecting leaves of all ages in 
the Madison fields, while at Ripon only the older leaves at the hill centers 
showed the angular spots. 

The relation of wind to dissemination by rain spattering has not been 
studied experimentally, but the comparison of the way the disease 
spread in differently situated fields throws some light on the question. 



2IO 



Journal of .Agricultural Research 



Vol. XV, No. 3 



One field, which we may call field A (fig. 2), was on the southeast slope of 
a hill and surrounded by trees so that it was well protected from wind, 
especially northwest wind. Field B was on the west slope of another 
hill and freely exposed to wind. The original centers of angular-leafspot 
in B were on the north side near the top. Thunder showers on July 12, 
16, and 19 were accompanied by high winds from the north and west. 
On July 29 it was noted that in field A the infested areas were strikingly 
more delimited than those in field B. The difference could be explained 
only as a result of the difference in exposure to winds. 

Another field was situated on a freely exposed west slope, and its rows 
ran with the hill, east to west. No notes on disease distribution there 
were taken until August 4. On that date there was a center of abundant 



* ****** 





* * * * /9ifZZ? & 

Fig. 2. — Diagrams of cucumber fields to show relation of -wind and drainage water to angular-leafspot 
dissemination. Double circles indicate the location of original centers of infection prior to storms. 
Small circles represent secondary infections due to storms. In field A drainage water only was an im- 
portant factor because of protection from wind while in the freely exposed field B wind also played a 
part. Contour lines indicate elevation above level of Lake Mendota, Madison, Wis. Broken lines show 
the direction of the rows. For full explanation see text. 

infection in the seventh row from the north side and scattered infections 
in all of the 16 rows south of it. North of this badly infested area the 
adjacent row, the sixth, showed a very small number of infections, and 
the 5 others were entirely free from the disease. Obviously the northwest 
winds had played an important part here also in spreading the disease. 
Faulwetter (7) has shown that wind in connection with rain is an im- 
portant factor in the spread of a similar bacterial disease, the angular- 
leafspot of cotton. 

The fact that the thundershowers mentioned as resulting in a marked 
spread of angular-leafspot occurred during the daytime supports the 
inference which may be drawn from the facts regarding the relation of 
stomatal movement to infection, that rains which occur in the daytime 
are more effective in the spread of the disease than are those occurring 
at night. 



Oct. 21, 1918 Angular-Leaf spot of Cucumber 211 

BY DRAINAGE WATER 

Evidence concerning the distribution of the causal organism by 
drainage water during rains was afforded by comparing developments in 
fields A and B which were mentioned in the preceding section. The 
rows in field A ran across the hillside, while in B they followed the direc- 
tion of the slope (fig. 2). After the rains of July 12, 16, and 19 the dis- 
ease appeared in field B throughout the length of the rows in which it 
had been noted earlier and in plots below them where disinfected seed 
had been planted. In field A, however, the spread of the disease was 
nof mainly along the rows but rather crossed the rows, following the 
path of the drainage water. The supposition is that the organisms were 
carried by the drainage water and from it were spattered by the rain to 
the healthy plants. Dissemination by drainage water has been noted 
before with fungus diseases — for example, cabbage-yellows by Jones 
and Gilman (9) — but, so far as is known to the writer, no evidence has 
before been published in regard to its significance in the case of a bac- 
terial disease. 

Attempts to prove that drainage water carried the causal organisms 
were made on two occasions late in the summer. Samples of drainage 
water caught during rains at the lower edges of infested fields were 
taken to the greenhouse and sprayed on healthy plants. The negative 
results are not surprising in view of the fact that few new infections 
developed in the fields where the water was caught, and that negative 
results from attempts to isolate the bacterium from beetles from these 
same fields also indicated that a large proportion of the bacteria had been 
killed as a result of the long, preceding period of dry weather. 

BY PICKERS 

The spread of diseases due to fungi has been attributed to pickers — 
for example, bean anthracnose, by Whetzel (17)— but, so far as the 
writer is aware, no such fact has been demonstrated for a bacterial dis- 
ease. Experiments in the case of the cucumber angular-leafspot have 
shown that the disease may be spread by pickers if picking is done 
when the exudate is present on the infected leaves. On August 8 and 9, 
1 916, the matter was tested as follows: At 5.30, 7.30, and 8 o'clock on 
the morning of the first day and at 8 a. m. on the second day two or 
three leaves in each case were inoculated by rubbing with the hands 
(as is done by pickers) after having first rubbed them through the 
exudate on diseased leaves. In all four cases inoculated leaves became 
infected (PI. 15, B), while the uninoculated controls remained healthy. 

Picking is, of course, frequently done early in the morning and on 
rainy days when the leaves are wet and the bacterial exudate is abundant. 
Numerous observations show that the spread of the disease in the way 
described in the preceding paragraph often results. The most obvious 



212 



Journal of Agricultural Research 



Vol. XV, No. 3 



of the cases that have come under the writer's notice seems worth men- 
tioning in detail. At Princeton, Wis., a patch of cucumbers of seven 
rows was visited on August 12, 1916. In the middle of the third row, 
counting from the north side, there was a circular area of diseased 
leaves, badly shattered by the rain of August 10. West of this area of 
shattered leaves no new infections were evident. East of this area, 
however, there were numerous recent infections, and the number of 
these varied nearly inversely as the distance from the original center. 
The location of diseased leaves and the position of the spots on them 
corresponded to observations on dissemination by pickers made in 
other places. When passing the patch later in the day the owner was 
seen starting to pick on the west end of the first row, so that when he 
would come to pick the third row he would be working eastward (fig. 3). 
Evidently he must have followed the same course when the bacterial 
exudate was abundant enough to thoroughly contaminate his hands. 



®-©-e- 



Fig. 3. — Diagram of cucumber field to illustrate picker dissemination of angular-leafspot. ®— original 
center, = new infections, arrows indicate direction the picker worked. 

BY INSECTS 

Cucumber beetles (Diabrotica vittata Fab. and D. duodecimpunctata L.) 
have repeatedly been seen crawling over infected leaves and flying 
about the fields when the bacterial exudate was plentiful, as early as 
5.30 a. m. Platings from these insects were made in only a few cases. 
That some of them would crawl through exudate and become con- 
taminated seemed unquestionable. In one instance platings from a 
water blank, in which had been dropped three 12-spotted beetles, yielded 
the causal organism. 1 Bees have been observed visiting the plants as 
early as 7.30 a. m. and have been seen to occasionally brush against 
exudate-bearing leaves. 

The carrying of the causal bacteria from one part of a field to another 
by insects is no doubt significant, but in view of the other ways in which 
the local spread is accomplished it is far less important than is the dis- 
semination of the organism by the same agency from diseased to healthy 

1 The organism in this case was not tested as to pathogenesis, but was identified by colony characters 
on potato-dextrose agar. These typical colonies were more numerous than any other kind, and by transfers 
(unintentionally delayed until 10 days had passed) it was shown that the organisms were dead, as is true 
for the angular-leafspot organism with media containing dextrose. 



Oct. 21, 1918 Angular-Leaf spot of Cucumber 213 

fields. The evidence supporting the latter idea is observational. During 
1 91 6 six experimental fields were grown near Madison in the same 
vicinity with four privately owned commercial fields. The distances 
between the commercial and the experimental fields varied from about 
30 rods to % mile. Angular- leaf spot appeared in all six of the experi- 
mental plots early in July. It appeared in only three of the others and 
in these not until nearly the middle of August. Prior to this, in the fore- 
part of the month, there was a period in which there was an abundance 
of bacterial exudate in the infested fields and when insects, especially 
the beetles, were very active. It is of interest here to note that in no 
case did the original center of infection in a private field develop at the 
edge, but rather in the interior of each of the three patches. It may be 
stated with confidence that during the time in question no one except 
Dr. M. W. Gardner and the writer visited both the experimental and 
commercial fields, and as these visits were made when the leaves were 
dry there seems little probability of the organisms having been trans- 
ported by us. Comparable developments were observed at Ripon, 
Princeton, and Pittsville, Wis. 

Closely correlated with picker dissemination and the probability of 
spread by insects is the relation of atmospheric humidity to the disease. 
Under conditions of high relative humidity, such as frequently prevail 
on summer nights, the invaded areas of the leaves take on clear-cut 
angular shapes and the bacterial exudate becomes abundant. (See PI. 
13, A). Such nights were those of August 8 and 9, 1916. The relative 
humidity, as recorded by a Friez hygrograph, varied on those nights 
from 74 and 80 per cent, respectively, at 7 p. m. to 90 per cent, where it 
continued until 6 a. m. Observations in the early mornings showed 
abundant signs, as described, that conditions for the progress of the 
disease had been most favorable. In steam-heated greenhouses, with 
the relative humidity varying from 45 to 60 per cent, the disease develops 
poorly or not at all. 

OVERWINTERING 

The several ways in which the causal organisms of other bacterial dis- 
eases have been thought to pass the winter have been kept in mind in 
searching for evidence as to how the angular-leafspot bacterium over- 
winters. 

SOIL 

The sensitiveness of the organisms to freezing, as elsewhere recorded, 
renders doubtful the possibility of their living over the winter in the soil 
or in the debris of diseased vines in northern climates. A limited amount 
of work on this question indicates that the bacteria do not live for long 
periods in the soil. The question can not, however, be definitely settled 
until further study of it has been made. 



214 Journal of Agricultural Research voi.xv. No. 3 

INSECTS 

The hypothesis that the bacteria may overwinter in or on the bodies of 
insects is here mentioned because it might seem plausible in view of the 
theory advanced with good evidence by Rand and Enlows (13) to 
account for the overwintering of the organism causing the wilt of cucur- 
bits. No dependent relationship, such as has been found to exist between 
the wilt and cucumber beetles, has been observed in the case of angular- 
leaf spot. Field observations in 191 6 furnish some good negative evi- 
dence relative to the insect-overwintering theory. In one vicinity near 
Madison six cucumber fields on "new" land planted with seed from one 
source became diseased early, while four fields (planted with seed of a 
different source) on or very near land which had previously been planted 
with cucumbers did not develop the disease early. The early brood of 
beetles was fully as abundant on the four latter fields as on those six 
that became diseased early in the season. 

SEED 

The observations which formed the preliminary basis for the seed- 
overwintering theory have been printed before (6), but for the sake of 
bringing together all the pertinent evidence may be here repeated. In 
June, 1915, angular-leafspot was observed in abundance in a field south 
of Portsmouth, Va. The plants were developing their fifth and sixth 
leaves at the time. The field was on newly cleared land, surrounded by 
woods and at least 3 or 4 miles from the nearest cucumber patch. The 
evidence pointed strongly to the introduction of the organisms with the 
seed. 

The developments in the fields near Madison in 191 6 gave further 
evidence that the organisms are introduced with the seed. The six 
experimental fields previously mentioned were all on land which had not 
been planted to cucumbers for at least three years. Angular-leafspot 
appeared on seedlings in all six of these fields, and in three of them it 
was noted on the cotyledons. In the case of the four commercial fields 
near by which were planted with seed from another source the disease 
did not appear at all in one and not until late in the season in the other 
three. This evidence so strongly indicated that the bacteria live over 
winter on the seed that it seemed worth while to study the matter in the 
commercial seed fields. Accordingly the writer visited a large seed- 
producing center in Iowa and Dr. M. W. Gardner, because of his interest 
in the question in relation to cucumber anthracnose, visited a seed farm 
in Ohio. In one of the seed fields in Iowa the disease was widespread 
and, according to a hasty estimate, 25 per cent of the fruits were attacked. 
Dr. Gardner found spots on the fruits in the Ohio fields which he was 
reasonably sure were due to the angular-leafspot organism. 



Oct. 21. 1918 Angular -Leaf spot of Cucumber 215 

Since the fruit invasions are local and shallow, it is evident that the 
seed rarely, if ever, becomes attacked naturally. A study of the way the 
seed is thrashed, however, sheds further light on the way in which the 
seed may become contaminated. 

The thrashing process practiced on the farms visited is probably in 
general use. It is begun by shoveling the whole fruits into a grinding 
machine which chops them up and allows the larger parts of the fruit 
pulp to be carried off on a rotating screen. The seed, the juice, and the 
smaller pieces of pulp fall through the screen and are drained into con- 
tainers. This much of the process would doubtless afford ample oppor- 
tunity for the organisms to reach the seed. The next step, however, 
probably increases the chances for the seed to become contaminated. 
The seed with the pulp and juice is left in the barrels with frequent 
stirring for a period of time varying usually from one to three days. The 
angular-leafspot organism doubtless multiplies rapidly in this well 
aerated mixture of juice and pulp unless conditions become unfavorable 
owing to the by-products of other organisms. After the material con- 
taining the seed has stood in the barrels for the time mentioned, it is 
poured into other containers and the seed separated out as well as pos- 
sible by repeated washings with water. Then the seed is dried on shallow 
trays, at first in the sunlight and later indoors. The process of thrashing 
includes no step which would be likely to kill all the bacteria. 

Seed for further study was sent to Madison from both the Iowa and 
Ohio farms. The details of some of the experiments performed with 
this seed and the results are here summarized. 

Experiment op February 20, 1917. — Sixteen flats of sand were steamed at 7 pounds' 
pressure for one hour. Then each flat was planted with approximately 150 seeds 
from Iowa. Before touching the seed the hands were rinsed in 70 per cent alcohol as 
a precautionary measure. After the flats were planted they were wet down with 
water that had been boiled (cooled), and boiled water was used in all subsequent 
watering. None of the resulting seedlings were diseased. 

Experiment op March 3, 1917. 1 — Fourteen flats of sand were steamed as before. 
The hands were disinfected with mercuric chlorid and alcohol. The trowel was 
treated with hot water. Twelve flats were planted with seed from the lot from Ohio, 
about 100 seeds to each flat. One flat was planted with seed from the 1915 supply 
which had been treated with 1 per cent formaldehyde for 20 minutes and another 
with seed from the same lot which had been treated with 1 to 1,000 mercuric chlorid 
for five minutes — these two for controls. The flats were covered with sterilized wire 
screen to protect them from mice and rats. The flats were watered with water (cooled) 
which had been boiled. 

On March 19 the writer found four seedlings in one of the flats showing typical signs 
of angular-leafspot as they had been observed on seedlings artifically infected by plant- 
ing inoculated seed and on naturally infected seedlings in the field (PI. 15, A). The 
attacked seedlings were in two separated places — two affected seedlings next to each 
other in each case — and apparently one seedling had been infected from its neighbor 
in each instance. From a seedling from each of the two placesthe organism was isolated 
and used in pure culture inoculations to reproduce the disease. Stained sections 

'Performed by Dr. M. W. Gardner in connection with his work on cucumber anthracnose. 



216 Journal of Agricultural Research vol. xv, No. 3 

of one of the spots on one of the cotyledons showed bacteria in the intercellular spaces. 
Two of the seedlings were preserved as herbarium specimens. On March 27 another 
infected seedling was noted in another flat. The organism was isolated and identified 
by inoculation as before. 

Experiment op March 27, 1917. 1 — Fourteen flats of sand and four of heavily com- 
posted garden soil were steamed for one hour at 7 pounds' pressure. All but two were 
planted with the seed from Ohio. These two planted with seed treated in 1916 with 
mercuric chlorid and untreated seed from the 1916 supply, respectively. Precau- 
tionary measures taken as before. On April 4 a typically infected seedling was noted 
in one of the sand flats planted with the Ohio seed and on April 7 a well-advanced 
stage of the disease was discovered on a seedling in another sand flat of the Ohio seed. 
There was no doubt as to the cause of the lesions from the characteristic signs — viz, 
water-soaked tissue and white exudate residue. Platings from each of these seedlings 
gave an abundance of the typical colonies. 

The results of these experiments and the fact that in Dr. Gardner's 
later tests of the Ohio seed in sterile damp chambers one seedling in each 
of two damp chambers developed the typical signs of the disease prove 
that the angular-leafspot organisms may live for at least seven months 
on the seed. There seems no reason to doubt but that they can survive 
for two months longer and infect the seedlings as field observations have 
indicated. 

The use of seed as badly contaminated as the lot from Ohio was found 
to be, would have resulted in the early development of angular-leafspot 
in as large a proportion of the fields as occurred in 191 6 in Wisconsin. 
From the Ohio lot approximately 3,500 seeds were planted with the pre- 
cautions described. Seven, or a proportion of 1 to 500, of the resulting 
seedlings developed angular-leafspot. With this proportion or 0.2 per 
cent and the use of 2 pou'nds of seed per acre, as is usually practiced, there 
would be about 72 plants infected from seed-borne organisms to every 
acre of cucumbers. 

As to how the organisms are protected on the seed so as to withstand 
the long period of desiccation there is no conclusive evidence. It seems 
most likely to the writer, however, that they get in at the microp3 r lar end 
of the seed, and so are protected within the seed coat. The fact that the 
infections of the seedlings nearly always occur on the edge of the coty- 
ledons near the point of attachment to the stem — the part of the coty- 
ledons which is at the micropylar end — indicates that the bacteria are 
probably harbored beneath the seed coat (PI. 16, A). It might be 
argued that, since on germination the attached ends of the cotyledons 
are the first to emerge, the portion which becomes infected is the first 
part which is exposed to organisms on the surface of the seed. This ex- 
planation, however, seems less probable to the writer- than that the or- 
ganisms are sheltered inside the micropyle. At any rate subsequent 
work by Gardner and Gilbert (8) has shown that the bacteria are so lo- 
cated that they can be killed by chemical treatment of the seed. 

1 Performed in cooperation with Dr. M. W. Gardner. 



Oct. 21,1918 Angular-Leaf spot of Cucumber 217 

REMEDIAL MEASURES 

The matter of finding some means of controlling angular-leafspot has 
been kept in mind in all the studies, especially in comparing cucumber 
varieties as to susceptibility, in observing the ways in which the disease 
is spread, in testing the sensitiveness of the organism to desiccation, to 
heat and chemical germicides, and in trying to determine how the bac- 
teria are overwintered. 

RESISTANT VARIETIES 

Tests made in the field in 191 5 and 191 6 by growing the horticultural 
varieties (listed on page 206) where they were exposed to infection yielded 
no encouraging results. There was no marked difference in susceptibility 
between the varieties. No instance of individual resistance has been 
observed in all the fields which have been examined. 

SANITATION 

The evidence recorded under the section on dissemination by pickers 
justifies the recommendation that where feasible the picking of fields into 
which the disease has been introduced be done at times other than in 
early mornings or on rainy days when the bacterial exudate is abundant. 
In cases where it is necessary to pick over a partly diseased field under 
those unfavorable conditions it may be worth while to pick the healthy 
part of the field first. 

The hope for the complete control of the insect pests, particularly the 
cucumber beetles, seems to be a thing for which there is little basis. The 
fact, however, as discussed under the consideration of dissemination, 
that there is good evidence that these insects are instrumental in spread- 
ing the disease from one field to another makes more urgent the need of 
finding better ways of holding them in check. 

SPRAYING 

Spraying experiments in which Bordeaux mixture (3-6-50) was the 
principal fungicide used, were under observation in Wisconsin during the 
summers of 19 14, 191 5, and 191 6. Noticeable checking of the disease 
resulted each year. Yield results were in all cases so vitiated by factors 
other than the spraying, especially the mosaic disease, that comparisons 
of them were of little value. Furthermore, the disease did not develop 
in the most destructive way on the experimental fields. The data at 
hand therefore hardly justify a definite statement of the value of spray- 
ing for this disease, but, in the opinion of the writer, the practice would 
not in Wisconsin and neighboring States be generally profitable on a com- 
mercial scale. Several reasons have furnishd the basis for this conclu- 
sion. Because of the early appearance of the disease, spraying, to be 
most effective, would have to be started nearly as soon as the plants 
came up. Because of this need for beginning early and continuing the 



218 Journal of Agricultural Research vol. xv, x . 3 

spraying at frequent intervals throughout the season, the cost would 
probably be greater than could be compensated by the resulting increase 
in yield. Cucumber vines normally grow so rapidly that the intervals 
between spraying would have to be short in order that a considerable 
portion of the younger leaves would not be exposed to infection a good 
deal of the time. The fact, however, that the disease is mainly depend- 
ent on rain for dissemination and that long, rainless periods occur at ir- 
regular times would make it a hard matter to recommend a spraying 
schedule which would be economical. 

Spraying where profitable because of other considerations has no 
doubt been of increased value because of the partial protection afforded 
from angular-leafspot damage. 

Burger (4) reported beneficial results from spraying for this disease 
on the basis of a limited amount of spraying in one season. He found a 
decidedly smaller number of infected fruits in the sprayed than in the 
unsprayed plots, and reported that the leaves in the sprayed plots were 
healthier than those in the check row r s. It is interesting to note, how- 
ever, that his recorded yields show that in every case the total yield, 
including infected and healthy fruits, was greater from the check than 
from the sprayed plot. This fact may be correlated with the unsettled 
question of spray injury to cucumber. 

The readiness with which angular-leafspot is spread b)r spattering of 
rain makes a spraying experiment, in which the check rows are parallel 
and adjacent to those sprayed, incomparable to the spraying of a whole 
field. This fact should be borne in mind when further spraying tests 
are made. 

SEED TREATMENT 

The evidence indicates strongly that the angular-leafspot organism 
overwinters principally on the seed. If this be true, the matter of con- 
trolling the disease is greatly simplified, especially from the standpoint of 
the industry of growing cucumbers for pickling. Some of the pickle 
companies grow their own seed, while others buy seed from seedsmen. 
All companies, so far as is known to the writer, furnish the seed to the 
growers with whom they contract to raise the cucumbers. There will 
be little difficulty, therefore, in getting the seed disinfected before it is 
distributed to the farmers, after a satisfactory method of treatment has 
been worked out. 

Preliminary tests of treatments with hot water and with chemical dis- 
infectants have been made. Seed has been treated as follows : Soaked in 
water at 50 and 52 ° C. for 10 minutes; in formalin (4 per cent) for 
5 minutes and 2 minutes; in copper sulphate (1 per cent) 10 minutes and 
5 minutes; and in mercuric chlorid (1 : 1 ,000) for 5 minutes and 2 minutes. 
These tests were run on such a small scale because of limited greenhouse 
space for testing germination that conclusions can not be drawn as to 



Oct. 21, 1918 Angular-Leaf spot of Cucumber 219 

the effectiveness of the treatments in killing the causal organism, but 
they do indicate that no important injury * to the seed from these treat- 
ments may be expected. Extensive field tests with treated seed and 
further field trials of disinfectants with special reference to injury to the 
seed are under way in Wisconsin, Michigan, and Indiana under the super- 
vision of Mr. W. W. Gilbert and Dr. M. W. Gardner. 

SUMMARY 

Angular-leaf spot of cucumber was first noted in Wisconsin in 191 4 
and its bacterial nature established in 191 5. The disease is the same as 
that described by Smith and Bryan (15) and earlier reported by Burger 
(2), Traverso (16), and Potebnia (12). 

The disease is probably world-wide in its distribution. Under favor- 
able meteorological conditions it does a good deal of damage. Because 
of its widespread and frequent occurrence it should be ranked among 
the cucumber diseases of major economic importance. 

Leaf infection is stomatal. Inoculations made at different hours 
showed that infection occurs chiefly during the day rather than the 
night. This is probably to be explained by the fact that the stomata 
are open during the day and closed at night. 

Fruit infection is stomatal. The disease first appears there as small, 
localized, circular, water-soaked spots. The centers of the spots later 
become whitened, so that they are more readily noticed. 

Rain is the most important means of dissemination, but pickers and 
probably insects play a part in this process. 

The causal organism is sensitive to desiccation, is readily killed in 
artificial media by freezing, is killed in liquid media by an exposure for 
10 minutes at 50 C, and is readily killed by dilute solutions of formal- 
dehyde, copper sulphate, or mercuric chlorid. The sensitiveness of the 
organism to these chemicals is increasingly greater in the order mentioned. 

There is substantial evidence that the causal bacteria overwinter with 
the seed. 

No marked variation in resistance or susceptibility has been found 
among horticultural varieties of cucumbers. A few ornamental gourds 
are attacked by the disease. Attacks are limited to the cucurbits, and 
in that family no important crop plant other than the cucumber has been 
found affected. 

Sanitary measures, such as precautions in picking and in control of 
insects, may be helpful. Spraying with Bordeaux mixture checks the 
disease, but is of doubtful value as a general commercial procedure in 
regions where spraying would not otherwise be practiced. Seed treat- 
ment offers the greatest hope of satisfactory control. 

1 In the subsequent field tests carried on by Gilbert and Gardner, the 4 per cent formalin treatment 
caused considerable injury to cucumber seedlings, resulting in marked rolling of cotyledons and retarda- 
tion of growth. The mercuric chlorid treatment(i: i.oooforfive minutes) has proved safe and effective (8). 



220 Journal of Agricultural Research vol. xv, no. 3 

LITERATURE CITED 
(i) Burger, O. F. 

1913. A bacterial ROT OF cucumbers. Phytopathology, v. 3, no. 3, p. 
169-170. 



1913. A NEW cucumber disease. In Fla. Agr. Exp,. Sta. Rpt. [i9ii]/i2, 
p. c-ci. 



1914. bacterial rot OF cucumbers. In Fla. Agr. Exp. Sta. Rpt. [i9i2]/i3, 
p. xc-xciv. 



(2) 

(3) 

(4) 

1914. cucumber rot. Fla. Agr. Exp. Sta. Bui. 121, p. 97-109, fig. 37-42. 

(5) CarsnER, Eubanks. 

1916. ANGULAR LEAF-SPOT, A BACTERIAL DISEASE OF CUCUMBERS. (Abstract.) 

In Phytopathology, v. 6, no. 1, p. 105-106. 
(6.) 

191 7. DO THE BACTERIA OF ANGULAR LEAF SPOT OF CUCUMBER OVERWINTER 

on the SEED? (Abstract.) In Phytopathology, v. 7, no. 1, p. 61-62. 

(7) Faul WETTER, R. C. 

1917. DISSEMINATION OF THE ANGULAR LEAFSPOT OF COTTON. In Jour. Agr. 

Research, v. 8, no. 12, p. 457-475, folded chart. Literature cited, 

P- 473-475- 

(8) Gardner, M. W., and Gilbert, W. W. 

1918. CUCUMBER ANGULAR LEAF-SPOT AND ANTHRACNOSE OVERWINTERING 

and SEED TREATMENT CONTROL. (Abstract.) In Phytopathology, 
v. 8, no. 2, p. 70-80. 

(9) Jones, L. R., and Gilman, J. C. 

1915. THE CONTROL OF CABBAGE YELLOWS THROUGH DISEASE RESISTANCE. 

Wis. Agr. Exp. Sta. Research Bui. 38, 70 p., 23 fig. Literature cited, 
p. 69-70. 
(10) Lloyd, F. E. 

1913. LEAF WATER AND STOMATAL MOVEMENT IN GOSSYPIUM AND A METHOD 
OF DIRECT VISUAL OBSERVATION OF STOMATA IN SITU. In Bui. 

Torrey Bot. Club, v. 40, no. i, p. 1-26, 3 fig. 
(n) Pool, Venus W., and McKay, M. B. 

1916. RELATION OF STOMATAL MOVEMENT TO INFECTION BY CERCOSPORA 

beticola. In Jour. Agr. Research, v. 5, no. 22, p. 1011-1038, 6 fig., 
pi. 80-81. 

(12) Potebnia, A. A. 

1915-16. GRIBNYE PARAZITY VYSSHIKH RASTENII KHAR'kOVSKOI I SMEZHNYKH 
GUBERNII (FUNGI PARASITIC ON HIGHER PLANTS IN KHARKOV AND 

neighboring districts). [Pub.] Khar'kovsk. Oblastn. Sel'skokhoz. 
Opytn. Stan. Phitopatol. Otd. 1, p. 1-250, illus. 

(13) Rand, F. V., and Enlows, Ella M. A. 

19 1 6. TRANSMISSION AND CONTROL OF BACTERIAL WTLT OF CUCURBITS. In 

Jour. Agr. Research, v. 6, no. 11, p. 417-434, 3 fig., pi. 53-54. 

(14) Smith, Erwin F. 

1911. bacteria in relation to plant diseases, v. 2, Washington, D. C. 
(Carnegie Inst. Pub. 27, v. 2.) 

(15) and Bryan, Mary K. 

1915. angular leaf-spot of cucumbers. In Jour. Agr. Research, v. 5, no. 

n, p. 465-47°. P 1 - 43-49- 

(16) Traverso, G. B. 

1915. sulla bacteriosi del CETriolo in italia. In Atti R. Accad. Lincei, 
Rend. CI. Sci. Fis., Mat. e Nat., s. 5, v. 24, sem. 1, fasc. 5, p. 456-460. 

(17) Whetzel, H. H. 

1906. some diseases of beans. N. Y. Cornell Agr. Exp. Sta. Bui. 239, p. 
195-214, illus. 



PLATE 13 

A. — Cucumber leaf five days after inoculation with Bacterium lachrymans, showing 
severe infection. The dark, angular spots had a water-soaked appearance. Drops 
of bacterial exudate may be seen on some of the spots. Photographed by Mr. Fred 
R. Jones. 

B. — Plant a, photographed seven days after inoculation with Bad. lachrymans 
shows considerable stunting as compared with the uninoculated control, plant b. 
Young plants as severely attacked have often been seen in the field. 



Angular-Leafspot of Cucumber 



Plate 13 




Journal of Agricultural Research 



Vol. XV, No. 3 



Angular-Leafspot of Cucumber 



Plate 14 




Journal of Agricultural Research 



Vol. XV, No. 3 



PLATE 14 

Stomatal movement in relation to infection. The cucumber leaves used in the 
experiment were of the same age and on similar plants. So far as possible, conditions 
of inoculation were similar except that leaf a was inoculated with Bacterium lachry- 
mans at 9.15 a. m. and leaf b at 6 p. m. 



PLATE is 

A. — Overwintering on seed: Natural infections on cotyledons of seedling grown in 
steamed sand from commercial seed which had been kept in storage for seven 
months after harvesting. Experiment of March 3, 1917. Enlarged about \% times. 

B. — Picker dissemination. Infection resulting from inoculation of a cucumber 
leaf at 7.30 a. m. by rubbing with the hand immediately after touching diseased 
exudate-bearing leaves. 

C. — Dissemination by rain. The older leaves in the center of the row were badly 
infected during a rainy period. Young leaves on the sides of the row which developed 
during a rainless period are comparatively free from the disease. Photographed by 
Mr. W. W. Gilbert. 



Angular-Leafspot of Cucumber 



Plate 15 




Journal of Agricultural Research 



Vol. XV, No. 3 



Angular-Leafspot of Cucumber 



Plate 16 





Journal of Agricultural Research 



Vol. XV, No. 3 



PLATE 16 

A. — Seedling infection, resulting from seed inoculation with Bacterium lachrymans. 
Seeds were wet with a pure culture of the angular-leafspot organism and planted in 
sterilized soil. Note location of cotyledon infections. Photographed 14 days after 
planting. 

B. — Cucumber fruit showing small, watersoaked, circular spots with white centers 
resulting from natural infections with angular-leafspot. 



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